The Formation of Rims on Dolomite Aggregate in Iowa Highway Concrete

Abstract

<p>Rims have formed on the margins of dolomite (Ca,Mg(C03 ) 2 ) aggregate in Iowa highways due to dedolomitization reactions between aggregate and concrete paste. These dedolomitization reactions are usually accompanied by a volume change, due to a conversion of dolomite to a calcitic dolomite, that results in the development of micro-cracks in the aggregate and the concrete paste. These cracks become channelways for solutions, often saline in nature due to the application of deicing salts, that result in further deterioration of highway concrete. Rim development on aggregate particles commonly results in highway concretes with short service lives. The most complete sequence of rims on aggregate in Iowa highway concretes occurs where individual reaction zones are up to 200 μm wide and consist of an inner dark-colored dolomite rim, an outer light-colored dolomite rim, and an outer light-colored Portland cement paste rim. These rims are superimposed on unaltered dolomite aggregate particles and cement paste. Nondurable concretes (service life of< I 6 years) exhibit better developed and more abundant rims and a higher density of microcracks and contain dolomite with a more variable grain size and higher porosity than durable concretes (service life of > 40 years). The rims of lightcolored dolomite aggregate show an increase in Ca content and a concomitant decrease in Mg adjacent to the aggregate-paste interface, along with the formation of calcite, brucite, and portlandite. The ability to distinguish dolomites that are susceptible to deterioration from those that are less likely to deteriorate should save expenditures since highway maintenance costs will be reduced.</p>